The Conservative party's latest focus on tax cuts has been questioned by investors and economists who were leading supporters of the party's earlier tough line on reducing the national debt.

Though business leaders have recently queued up to back Tory proposals limiting planned rises in national insurance, a perceived lack of detail on how to pay for it is leading to unease among some of the City's so-called "deficit hawks".

Tim Besley, a former member of the Bank of England monetary policy committee who led a letter of concerned economists in the Sunday Times, said too much "election froth" over tax reduction risked distracting voters from the importance of repairing the public finances.

"Once the chips are down and we have a newly elected government in place, staring at the reality of deficit reduction, they are going to have to make their numbers add up," he told the Guardian.

"Sums like £6bn sound large, but they are not huge compared to the scale of the fiscal challenge. There is no substitute for a credible, independently risk-assessed plan [for cutting the deficit], so that we don't get knocked off course once we meet fresh challenges down the road."

Bond investors are also anxious that the attention paid to tax cuts in recent days will make it harder for an incoming government to take the difficult steps needed after the election – particularly if voting is close and results in a hung parliament.

Michael Amey, UK porfolio manager for Pimco, the world's largest bond investor, said: "When we see statements focusing more on what will be given away than what will be taken back, it is not good news from a market perspective. It means that if the election result is tight then the politicians appetite for tackling the deficit will be dampened.

The NI statements from a bond market perspective are a bad piece of news – at the margins it's not helpful."

Pimco has been one of the leading voices on the perils of prolonged public deficits, famously warning that the UK economy was resting on a bed of "nitroglycerine".

Market jitters about Britain's public debt have intensified in recent days as a result of renewed worries about default in Greece, and the cost of insuring against a similar UK default have begun to climb again in the days since George Osborne announced his plan to reverse national insurance rises.

"From a market perspective, the most important thing is clarity on deficit reduction and we don't have clarity at the moment," added Amey. "Between now and the election we have to adjust for the electoral cycle."

Some fund managers fear that the Bank of England may have to put up interest rates to compensate if the Tories' efficiency savings prove unachievable.

"At the moment we are willing to give both parties the benefit of the doubt because we expect they will be forced to act," Amey told the Guardian, "but the scenario that is causing most concern is the prospect of a hung parliament".

Though the Tories previously focused almost exclusively on the deficit, the fierce debate over national insurance has exposed them to criticism from the hawks.

Besley, now a professor at the London School of Economics, added: "This is a debate which reflects short-term electioneering with the parties using this as a device to make their values clearer to voters."

Several economists who signed the Sunday Times letter are known to be uncomfortable that it was seized upon by the Tory party in such a partisan way. Last week, another signatory, Professor David Newbery, of Cambridge University, said the Tories "don't seem to have a coherently worked-out" plan.

"I would have expected more clarity about what their long-term strategy is," he added. "One assumes that scaling down the public sector is one of those aims, but how they will do that we don't know."

Even the business community, which is much more fully behind Osborne, remains concerned about the deficit.

A spokesman for the Institute of Directors said most leaders of small and medium-sized business wanted an incoming government to press ahead with cuts in government spending.

He said the institute was disappointed in all three parties for their refusal to target specific Whitehall budgets for cuts. While tackling inefficiencies was welcome, he said, the parties should give more details.

"It is not just a case of efficiency savings, or even getting the private sector to do things the government currently does, but it is about saying what will no longer be provided and shrinking the size of the government," he said.

The institute, which has around 45,000 members, has argued against Labour plans to delay cuts until 2011/12.

"The argument that early cuts would jeopardise the recovery is mistaken. We believe that lower spending would have triggered a whole series of positive developments to assist growth," it said.

It's not every day that the leader of the Conservative party makes a direct appeal to readers of the Guardian. So, if David Cameron makes your hair stand on end, then hold your breath before you read these words which appear in a Guardian article by the Tory leader: